I own high mileage bricks (140k and 265k) and drive in Northern California where there are steep hills (but not mountains). I've tried various grades (10W-30 to 20W-50) over the years and notice that the temperature gauge increases (but doesn't hit red) when I go up steep hills. I read the FAQs section, but am still confused as to which Dino oil is "best" for my circumstances. Thanks in advance.

I agree with Jim. My daughter's 83 (145k mi) was showing a tendency to overheat when pulling hard on a warm day, despite semi-annual coolant flushes throughout the 20 yrs we've owned it. I pulled the rad (easy on a 240) and a rad shop tested and said it was almost 50% plugged with scale. After rodding out and reassembly ($85) everything's cool again.
I can't see how your choice of oil viscosity would contribute to overheating.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)

The factory recommendation is 10-30 for most temperatures. They recommend 15-40 only when you are pulling a trailer in hot temperatures. 20-50 is pretty thick, especially in the winter. I doubt your heat problems come from the oil choice. Try cleaning out the radiator fins. If that doesn't help, a radiator flush and coolant change with a new thermostat would be my next step.